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Newsgroups: bionet.molbio.evolution
Subject: Re: Flagellin and Tubulin Genes
Message-ID: <anthonyp-0910970931120001 at 137.131.4.62>
From: anthonyp at scripps.edu (Anthony J. Pelletier, Ph.D.)
Date: Thu, 09 Oct 1997 09:31:12 -0800
References: <61gvf8$5u7 at net.bio.net>
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In article <61gvf8$5u7 at net.bio.net>, Bob Cooper <rac7 at erols.com> wrote:
> Hi
>> I had a student question about genes that code for tubulin and
> flagellin. The flagella of prokaryotes and eukaryotes are analogous
> structures. Prokaryotic flagella are composed of flagellin subunits
> while the microtubules that make up the eukaryotic flagellum
> (undulopodium) are composed of tubulin. Does anybody know if the genes
> that code for these protein subunits (i.e., flagellin and tubulin) are
> at all homologous?
>> Bob Cooper
>rac7 at erols.com
I noticed that the majority of the responses you got had to do with the
proper use of the word "homology." For the record, I agree with the posts
that the word is misused grossly, and that there is an important
difference between homology and similarity.
But, that really doesn't get at your question...or at the students question.
Brian's very thorough posting of the gene sequence makes clear that there
is no evidence of a common ancestor.
Assuming you were interested not only in common ancestor, but also
similarity, I would like to add that the two proteins are really quite
different, both in structure and function.
The bacterial flagellum is really rather like a propeller, with a "rotor"
at the base of the flagellum in the membrane, which spins the flagellum in
one direction to move forward, and the other direction to "tumble" when a
change in direction is required. The rotor is a fascinating little
machine, which, to my knowledge, is the only case in which nature has
gotten close to inventing the wheel. It generates all the motion and the
flagellum is just a whip-like propeller.
The eukaryotic flagella and cillia are essentially identical structures to
each other in which a bundle of microtubules slide along each other to
deform the shape of the flagellum. The sliding is generated by another
protein, dynein, which works in a similar way to myosin, even though they
share no homology or similarity. By sliding along one another the
flagellum either undulates, or does a sort of breast-stroke motion (stroke
in one direction with the flagellum straight, recoil with it bent) in the
case of cillia, or the chlamydomonas flagella.
Since, for the eukaryotic flagellum, the motion is actually generated
along the flagellum itself, while for that of bacteria merely have to form
a rigid propeller, there are really few similarities in the functional
requirements of the structural proteins tubulin and flagellin. There also
are no simlarities to speak up in their structure.
-tony
Anthony J. Pelletier, Ph.D.
Assistant Professor, Department of Cell Biology
The Scripps Research Institute
La Jolla, CA
anthonyp at scripps.edu
